1. Field of the Invention
This invention relates to a process for the recovery of geothermal energy and mineral resources in the earth's zone of weakness in which large fractured heating zones in hot rock in the deep zone of the continental lithosphere are created by hydrostatic pressure at the bottom of a water column in a lined boring.
2. Description of Prior Art
The so-called Hot-Dry-Rock Process, developed in the 1960s by scientists at the Los Alamos laboratories in New Mexico, U.S.A. to create large heating surfaces in hot, dry, crystalline rock formations for the recovery of geothermal heat energy is known. The term "Hot-Dry-Rock Process" pertains to the goals of its inventors, namely, to create artificial, intercommunicating fracture zones in hot, dry rock formations in which no natural influx of surface or well water is available, in order to recover geothermal heat at locations where anomalies in the Earth's crust occur, but where no natural steam recovery is possible, such as in the traditional geothermally active areas of the Earth.
The Hot-Dry-Rock Process is based on the HYDRAFRAC Process, which has been used in the oil industry for about 30 years, to reactivate oil or natural gas wells which are drying up. In this Hot-Dry-Rock Process, the lower section of the boring, at which level a splitting of the rock formation is desired, is sealed off and subjected to high hydrostatic pressures which break up the surrounding rock by the HYDRAFRAC effect. The water pressure has to be clearly above the shear strength of the rock in order to form cracks with large heating volumes in the rock.
This process has been tested to depths up to 3000 meters in Fenton Hills, near Los Alamos, U.S.A. A further test at 4000 meters failed after a short initial success, due to failure of the pressure unit under the high applied hydrostatic pressure in the end section of the boring. The shear strength of the rock increases with increasing overburden. Thus, the Hot-Dry-Rock Process has limited applicability at depths where the shear strength of the rock is higher than the technically achievable pressures in the boring section sealed off by the so-called "packer." As has been observed in measurements in deep borings and in seismic studies, strong deviations from a generally linear increase in shear strength of the rock occur with increasing overburden. In certain strata, the increase in shear strength is over-proportional, reaching peaks of 2000 bars at depths of 3000 meters. Such unusually high increases further limit the range of applicability of the Hot-Dry-Rock Process. The most recent geological findings, seismic studies, and the results of the so-called Kola deep well on the Soviet Kola Peninsula, where a depth of 13,600 meters was reached after 17 years of drilling activity, prove that the shear strength of the rock forming the continental crust increases, with exception of a few local increases in certain formations, more or less linearly with overburden to a depth of about 13 kilometers. After 13 kilometers and up to a depth of about 30 kilometers, the rate of increase with increasing overburden then suddenly drops off. This behavior in crystalline rock is explained by the escape of crystalline water under the local temperature and pressure conditions. This initiates a natural Micro-Frac mechanism which fractures the previously compact rock and results in a strong decrease of the shear strength of the rock formation.
This finely fractured zone in the continental plates of the Earth's crust, with a reduced shear strength, is known in technical circles as the ZONE OF WEAKNESS. Temperatures in the upper portions of this zone reach about 500.degree. C. The temperature at depths of about 30 kilometers can be expected to be as high as 1000.degree. C.